1. Field of the Invention
This invention relates to digital-to-synchro conversion units for converting the digital input data from a DME (Distance Measuring Equipment) to synchro output voltages, and more particularly, it relates to digital-to-synchro converters of the type for providing continuous-real-time indications in angular form representing distance.
2. Description of Prior Art
With the advent and extensive use of digital circuits to implement systems that heretofore were primarily of the analogue type, a need has grown to provide digital-to-analogue converters. The indicators of the older type analogue equipment were obviously of the analogue type. In many aircraft installations, one need only replace both the analogue equipment and its associated indicator with modern digital equipment. There are cases, however, when this is not possible. The analogue indicator, for instance, may be part of a "flight director," an apparatus usually associated with an automatic aircraft controller that displays both the aircraft attitude and the outputs of the navigation instruments. The distance indicator for the flight director is usually an analogue unit that employs three synchro receivers. When a digital DME is used, the flight director is so expensive that it is cheaper to use a digital-to-analogue converter rather than remove and redesign its DME indicator.
Although the D/A conversion art is quite advanced satisfying the requirements of the avionics industry for units that are both accurate and inexpensive, there still remains difficult problems. Conventional units are large, expensive, and are complex. One solution, described in U.S. Pat. No. 3,662,379, uses tapped transformers and relays. Several transformers and relays are required for only one digit, noting that synchro indicators usually require three digits.
Another solution, described in the periodical "Electronics," Oct. 26, 1970, page 95, requires a conversion of the digital information-to-resolver signal as illustrated in FIG. 4 thereof. Resolver signals, however, are not compatible with synchros, and therefore, additional converter steps are required in order to develop the synchro signals. Known digital-to-synchro units require an input that is angle binary.. "Angle binary" relates angles by multiples of two. Thus, a rotation of 180.degree. is followed by 90.degree., 45.degree. steps in rotation for the most significant bit (MSB). The most significant bit (MSB) of such codes requires digital-to-digital converters at the input and a power type synchro device at the output. Also angle binary codes are not compatible with a conventional DME distance indicator since rotations are required in steps of either 36.degree. or 3.6.degree.. One must select, therefore, from the angle binary code those bits that provide the proper rotation. Such digital-to-digital converters would result in complex circuits. There is a need, therefore, for a digital-to-synchro converter that overcomes the problems of the known converters discussed above.